In passenger aircraft and transport aircraft construction aluminium fuselage shell segments are generally riveted together along longitudinal seams. This requires, in addition to an overlap between the components to be connected in the seam region, a large number of individual components that have to be used. Rivet connections of this type are thus expensive to produce and signify an undesired increase in weight in the seam region.
An alternative joining method for connecting thin-walled aluminium components of this type is known as friction stir welding. In this instance a rotating tool with a tool pin protruding from a tool shoulder is pressed into a connection region between two components to be connected until the tool shoulder rests on the component surfaces adjacent to the connection region. The components lie on a support. The shoulder heats the component surfaces adjacent to the connection region by friction, thus heating the material of the components to just below the melting point. The tool is then moved along the connection region whilst maintaining the contact pressing force, the tool pin mixing plasticised material in the connection region. In order to prevent the components from being welded to the support, the penetration depth of the welding tool is set in such a way that the connection region is not fully penetrated. The size of this remaining unpenetrated region of the connection region is to be maintained in the order of a few tenths of a millimeter. If the depth of the remaining region exceeds a tolerance limit, ‘penetration defects’ or ‘LOP’ (lack of penetration) defects may be produced owing to internal tensile stresses induced in the material and insufficient material plasticisation. Such LOP defects significantly reduce the fatigue limit of welds produced by friction stir welding.
However, when welding large components with long welds, for example as is the case when constructing fuselage and aerofoil sections, compliance with such narrow tolerances is hugely complex from a technical point of view and is therefore virtually impossible. It is possible for example to use a ‘bobbin friction stir welding tool’ in order nevertheless to eliminate the formation of LOP defects. In contrast to the conventional friction stir welding pin tool, this bobbin friction stir welding tool is applied to the connection region from two sides, thus ensuring that the connection region is always fully penetrated. In this instance however, the connection region must be accessible from each side. Furthermore, when using this tool the weld is visible from each side, which is not desirable for example when connecting two components which form a visible surface. Furthermore, the use of the bobbin friction stir welding tool means that the time and therefore cost involved is higher compared to the friction stir welding pin tool.
An alternative to the use of a bobbin friction stir welding tool is the use of a conventional friction stir welding pin tool in combination with a further method step in which the region with possible LOP defects is removed by machining, for example by milling. This means that the weld must not extend through a visible surface since ghost lines would then be visible. This finishing, by machining, of the connection region also involves considerable additional time.